1. Field of the Invention
The present invention relates generally to a computer system. Specifically, the present invention relates to a host adapter which reduces the processing demand on the local processor when processing input/output (I/O) requests from a host microprocessor.
2. Brief Description of the Related Art
In a computer system, a host microprocessor (host system) or central processing unit (CPU) retrieves information from a peripheral I/O device, such as a disk drive, by sending an I/O request to a host adapter. The host adapter processes the I/O requests intended for the peripheral I/O device. Generally, more than one I/O device is available to the host adapter. Further, most host adapters can simultaneously handle multiple I/O requests from the host microprocessor to multiple I/O devices. The host adapter includes a processor to process several low-level commands associated with each I/O request. For example, for I/O devices attached to a small computer system interface (SCSI) bus, a single I/O request may include commands to arbitrate for the SCSI bus, commands to select the target I/O device, commands to request data transfer, commands to disconnect and reconnect the I/O device from the SCSI bus, and other commands to change the SCSI bus state.
If the host microprocessor sends a large number of I/O requests, i.e., the I/O workload is high, the adapter processor needs to process a large number of requests. Before processing the requests, the frames containing the requests must be moved to memory used by the host adapter to store pending requests. Generally, the adapter processor is used to facilitate the transfer of the I/O requests from the host system to the host adapter memory area. In some configurations, the host system dedicates areas in host memory as a request queue and as a response queue. The request queue is used to facilitate the transfer of I/O requests from the host system to the host adapter. The response queue is used to facilitate the transfer of I/O command completion status information from the host adapter to the host system. In this configuration, the adapter processor is used to monitor and move frames to and from the queues in host memory. When a large number of I/O requests are issued and processed, the overhead (caused by the need for the adapter processor to move the data) can significantly reduce the throughput of the adapter processor in setting up the actual I/O operation. The reduction in throughput of the processor adversely affects the performance of the host adapter. As existing host microprocessors are designed to be faster, the slowdown in system performance, due to the adapter processor taking time to transfer and process the requests, is even more noticeable.
The rate at which peripheral I/O devices process I/O requests has increased along with the increase in host processor speed. The higher rate has led to a greater need for the host adapter to rapidly process I/O requests so as to fully utilize the faster I/O devices such as Fibre Channel devices. Thus, there has been a greater demand for the processing power used to process I/O requests and responses. Therefore, there is a need for a method of facilitating the transfer of I/O requests and responses between a host adapter and a host system, which reduces the load on the adapter processor.
One aspect of the present invention is a method of automatically loading I/O command blocks and I/O status blocks to and from buffers that are used by a processor on a host adapter to store I/O command blocks and I/O status blocks. The method for loading I/O command blocks to the adapter buffers monitors a request queue to determine when a new command is in the request queue. A buffer address is then retrieved from a list of available buffer locations. The data from the request queue is transferred from the host system to the adapter buffer at the location pointed to by the retrieved address. Once the data is transferred to the buffer, the address of the buffer is written to a filled buffer locations list. The adapter processor can then read the I/O command block by using the address from the filled list to retrieve the data from the buffer.
Another aspect of the present invention is a method for retrieving I/O status blocks from host adapter buffers. The method monitors a list of response buffers in adapter RAM, which contain I/O status blocks. An I/O status block buffer address is determined from a list of filled response buffers. The I/O status block is then transferred from the adapter RAM to a response queue in host memory. Finally, the buffer address is written to a free buffer location list to make the buffer available for another I/O status block. The adapter processor writes I/O status blocks containing response information to the buffers identified by the free list entries.